1. Field of the Invention
This invention relates to the preparation of isocyanate and/or isocyanate-based 1,3,5-triazine derivatives by the direct phosgenation of amino-1,3,5-triazines having at least three unsubstituted amino groups attached to the triazine ring(s).
2. Description of Related Art
Various derivatives of amino-1,3,5-triazines are described in the literature as being utilized in a wide variety of fields. An important use of certain of these derivatives, such as alkoxymethyl derivatives of melamine and guanamines, is as crosslinkers and/or reactive modifiers in curable compositions which contain resins having active hydrogen groups. While alkoxymethylated melamines and guanamines provide excellent results in a number of aspects, they also have the disadvantage of releasing formaldehyde as a volatile by-product under curing conditions. It has long been a desire of industry to find acceptable alternatives which do not emit formaldehyde upon cure.
One such alternative which has shown great promise is carbamate functional 1,3,5-triazines disclosed in the commonly owned U.S. Pat. No. 4,939,213, U.S. Pat. No. 5,084,541, U.S. Pat. No. 5,288,865, U.S. application Ser. No. 07/998,313 (filed Dec. 29, 1992), U.S. application Ser. No. 08/061,905 (filed May 14, 1993), and U.S. application Ser. No. 08/138,581 (filed Oct. 15, 1993), all of which are hereby incorporated by reference herein as if fully set forth. The carbamate functional 1,3,5-triazines disclosed in these references are believed to act in a manner similar to blocked isocyanates, and have been found to be particularly useful as crosslinkers in coating compositions based upon hydroxy functional resins, with the cured coatings possessing a wide range of desirable properties.
One impediment to the commercial use of these carbamate functional 1,3,5-triazines has been that known processes for their preparation have been somewhat cumbersome, difficult and expensive. For example, in previously incorporated U.S. Pat. No. 4,939,213 and U.S. Pat. No. 5,084,541, 1,3,5-triazine carbamates are produced in a two step process by first reacting an amino-1,3,5-triazine with oxalyl chloride to produce an isocyanate which is then converted to the corresponding carbamate by reaction with an alcohol. Further, in previously incorporated U.S. Pat. No. 5,288,865, carbamate functional 1,3,5-triazines are produced in a one-step process by reacting a haloamino-1,3,5-triazine with an acid halide. The primary disadvantages with these processes include the use of somewhat exotic and/or expensive halogenated starting materials and low ultimate yield of the desired products.
Many of these disadvantages have been overcome by the process disclosed in previously incorporated U.S. application Ser. No. 08/061,905, wherein carbamate functional 1,3,5-triazines are produced by reacting an at least bis-amino 1,3,5-triazine with an acyclic organic carbonate in the presence of a strong base.
Another process which overcomes many of these disadvantages is disclosed in previously incorporated U.S. application Ser. No. 08/138,581, wherein isocyanate functional 1,3,5-triazines are produced by the carbonylation of (halo)amino 1,3,5-triazines with carbon monoxide, in the presence of a metal promoter for promoting carbonylation.
It has now been surprisingly discovered that isocyanate functional 1,3,5-triazine derivatives can also be prepared from amino-1,3,5-triazines having at least three unsubstituted amino groups attached to the triazine ring, such as melamine, by direct phosgenation. These isocyanate functional 1,3,5-triazines may be further derivatized by contacting the same with a wide variety of well-known isocyanate-reactive materials. For example, these isocyanates may be readily "blocked" (for example, converted to the corresponding carbamate) by adding a blocking agent (such as a hydroxyl compound) to the isocyanate functional 1,3,5-triazine without isolating it. In addition, these isocyanates may be readily oligomerized by adding a multifunctional isocyanate-reactive compound (for example, a diol or diamine) to the isocyanate functional 1,3,5-triazine without isolating it.
It should be noted that it is generically known to obtain isocyanates by direct phosgenation of amines. It is, however, also well known that the amine functionality of amino-1,3,5-triazines, such as melamine, is not equivalent to other types of typical amine functionality. Significantly, melamines are among the least reactive of the "amines" and the most difficult to functionalize, and their behavior cannot normally be correlated to that of other known amines.
For example, most "typical" amines are highly reactive with acid halides. In a publication by E. M. Smolin and L. Rappaport entitled "S-Triazines and Derivatives," Interscience Publishers Inc., New York, page 333 (1959), it is reported that attempts to react an acid halide with the amino group on a 1,3,5-triazine such as melamine were not successful. Further, attempts to functionalize amino-1,3,5-triazines often results in substitution at the nitrogen on the triazine ring. For example, it is known that the reaction of melamine with alkyl halides, such as allyl chloride, results in alkyl substitution at the nitrogen on the triazine ring resulting in isomeamine derivatives.
Indeed, it is reported in U.S. Pat. No. 3,732,223 that the well-known phosgenation of amines fails to produce isocyanate functionality when applied to amino-1,3,5-triazines. In subsequent U.S. Pat. No. 3,919,221, the phosgenation of amino-1,3,5-triazines having one or two unsubstituted amino groups attached to the triazine ring to obtain monoisocyanate and diisocyanate triazines is reported to occur under certain specified conditions. Both of the above patents are incorporated by reference herein as if fully set forth.
It appears, however, that procedure generically described in previously incorporated U.S. Pat. No. 3,919,221 does not effectively or significantly proceed with amino-1,3,5-triazines having at least three unsubstituted amino groups attached to the triazine ring(s), such as melamine (2,4,6-triamino-1,3,5-triazine). Without wishing to be bound by any particularly theory, it is believed that solubility constraints of these (at least tris-unsubstituted amino)-1,3,5-triazines, in conjunction with the difficulties associated with the functionalization of amino-1,3,5-triazines in general, may hinder the reaction of such compounds with phosgene under the reaction conditions reported in the reference.
Surprisingly, a procedure has now been discovered in which phosgene (and phosgene sources) can readily and effectively be directly reacted with such (at least tris-unsubstituted amino)-1,3,5-triazines (direct phosgenation) to produce the corresponding isocyanate functional 1,3,5-triazine derivatives, which can further be readily and effectively reacted with known isocyanate-reactive materials (such as blocking agents) to produce the corresponding isocyanate-based derivatives thereof.